Fan Yang, 2 Hui Zhai, Fa Wang, and Dung-Hai Lee 5 1 Department of Physics,University of California at Berkeley, Berkeley, CA 94720, USA Department of Physics, Beijing Institute of Technology, Beijing 100081, P.R.China Institute for Advanced Study, Tsinghua University, Beijing, 100084, China Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (Dated: July 19, 2010)